MISSION PLANNING FOR THE ASTRO OBSERVATORY

"Mission Planning" is the process whereby the astronomical observations
desired by the science teams are turned into an actual operational plan
and observing schedule. Hence, it is composed of science
planning, where the scientists devise programs of investigation and
select the best astronomical objects to carry out the program, and
Operational Implementation, where a team of engineers and
technical staff produce the computer files and paper work that make the
plan happen. Clearly, the link between these two aspects of
mission planning is crucial for the success of a mission like Astro.
It is a complicated process that involves the interaction of the
individual science teams with each other, the science teams with
NASA/Marshall planners, the NASA/Marshall planners with NASA/Johnson
personnel (who are responsible for the space shuttle operations and
safety), and mission management personnel.

The mission planning process worked on Astro-1, but in a difficult way
that resulted in a lot of stress, especially with the launch delays
that affected that mission. A special Mission Planning Team was
constructed after Astro-1, composed of knowledgeable people from all
aspects of mission planning, to address the issue. The results were
tangible: communications were greatly improved, the process was
streamlined, and made more flexible at the same time. In addition, new
software was generated that not only made "pre-mission" planning more
efficient, but allowed tremendous flexibility for real-time replanning
during the Astro-2 mission itself.

Below we describe a few key aspects of mission planning for our
"Observatory on a Shuttle."

SCIENCE PLANNING

The astronomical observations planned for each Astro mission were
selected by the teams that dedicated years to building and
preparing the instruments, and/or Guest Investigators, who were
scientists that proposed to (and were selected by) NASA to have the
right to use the telescopes. Hundreds of hours were spent by
astronomers huddled in meetings selecting the highest priority observations
to be scheduled. Massive databases on all of the potential targets were
put together to assist in making these decisions. The programs of
investigation were selected mainly by the scientific interests of the
people involved, with consideration also given to the strengths of each
of the telescopes in the Astro package. Also crucial to this activity
was having computer programs that "simulate" what the instrument may
observe for a given target, based on the known information and
assumptions. Finally, when a launch date was known, the availability of
the targets were assessed, including such things as: a) Is the object
too close to the sun for observation? b) What is the target's
visibility with respect to the daylit and night time portions of each
orbit? c) Does the target's visibility change very much with time or is
it stable with respect to the assumed orbit? Only after all of these
and many other issues were addressed could a selection of potential targets
be assembled into a science timeline.

(There are a number of interesting differences in planning observations
for a shuttle-based telescope such as HUT versus a free-flying telescope
such as the Hubble Space Telescope. Click
here to see a brief description of
some of these effects.)

The science timeline is basically an orderly "wish list", where a
sequence of proposed science observations is laid out in an efficient
manner, using basic information about the target visibilities and space
shuttle constraints. It is the bridge between the science planning and
operational implementation phases of mission planning. The Astro
science teams plan this timeline using special software, meshing the
interests of all three science teams together into one timeline of
observations. This is the primary "product" delivered to the mission
planners at NASA/Marshall Space Flight Center.

OPERATIONAL IMPLEMENTATION

It is no small task to choreograph some 400 pointings of the space
shuttle at nearly 300 objects of interest to one or more of the
scientists. Suffice it to say that the science timeline is used as a
starting point, and then NASA mission planners turn it into a dazzling
variety of supporting information to make it all work. Guide stars and
roll angles for the Instrument Pointing System are selected for each
observation. A "maneuver timeline" for the shuttle is generated. Gimbal
angles to point the shuttle's communications antenna at one of the
Tracking and Data Relay Satellites are calculated. A "thermal profile" for the shuttle
is generated, along with many other safety-related checks. Detailed
crew schedules and crew procedures are generated, right down
to a minute by minute timeline for some activities. The list goes on
and on. Some of this work must be done in conjunction with NASA's
Johnson Space Center in Houston, which is responsible for the operation
of the Shuttle while in orbit. This process takes several months of
concerted effort by hundreds of people prior to a mission!

Meanwhile, the instrument teams are busy planning the detailed
instrument set-up procedures for each planned observation. There are
several important deliveries of information back and forth between the
MSFC planners and the instrument teams, all of which have to occur on
schedule to keep the mission planning process flowing properly. By the
time of launch, all of the necessary supporting materials are in place
to support the operational phase of the mission.

Given this description, it should be obvious that anything causing
disruption of these timelines of events could potentially cause great
difficulty for
mission operations.
If the shuttle failed to achieve the
anticipated orbit or launched two hours late due to a weather delay,
the detailed visibilities of the astronomical objects of interest all
change. Because of this, procedures for what is called "real-time
replanning" were implemented, whereby the timelines are regenerated
in 12 hour time blocks during the mission.
It was this real-time replanning capability, in conjunction with the
pre-mission work to assemble all of the supporting information, that led
to the tremendous scientific return from the Astro missions.